This disclosure relates to a photobioreactor for cultivating photosynthetic microorganisms. More particularly, this disclosure relates to a photobioreactor for photosynthetic organisms, in which the photobioreactor combines a large reactor volume with a short light path, which provides a large yield when photosynthetic microorganisms are cultivated.
A number of species of photosynthetic or phototrophic microorganisms, especially algae, are cultivated commercially. Photosynthetic microorganisms comprise a number of species, for example, but not limited to, Spirulina spp., Chlorella spp., Arthrospira spp., Dunaliella spp. and cyanobacteria.
Microalgae may be used as a food supplement given that they contain long-chained, polyunsaturated fatty acids, vitamins and antioxidants. They may also be used in the pharmaceutical industry given that some species contain pharmaceutically active substances, such as sterols, antimicrobial substances, antiviral substances and cancer-treating substances.
Photosynthetic microorganisms may also be used for energy production. By means of solar energy, green algae and cyanobacteria can decompose water into hydrogen and oxygen. Algae may be used as a source of biodiesel and are far more efficient for this purpose than that of traditional oil plants, for example oil palms.
Photosynthetic microorganisms are dependent on light as their energy source, CO2 dissolved in water as their carbon source, and nutrient salts in water as their source of mainly nitrogen, potassium, phosphorous and sulphur, and also trace minerals, for example iron, calcium, and magnesium.
Photosynthetic microorganisms may be cultivated indoors by means of artificial light, but most commonly the algae are cultivated outdoors in sunlight. The productivity, which is measured as biomass per unit of volume, depends on the availability of light and the light regime, among other things. The light regime refers to the time ratio between light and darkness.
There are a number of systems for cultivating microalgae. A simple cultivation method having low investment costs involves use of shallow ponds. A disadvantage of these ponds is that the microorganisms at the surface receive much light, whereas cells located a few centimetres further down in the water column receive less light. When the culture becomes dense, cells further down in the water column will receive very little light. They will therefore not grow. This may be remedied to a certain degree by stirring the water, insofar as turbulence will allow more cells to become exposed to sunlight.
Other systems for cultivating microalgae comprise pipe systems, formed either from straight pipes or curved pipes, as disclosed in patent documents ITF950093, WO 2008010737, GB 2118572 and U.S. Pat. No. 3,955,317, and by e.g. Carlozzi and Torzillo, 1996, (Productivity of Spirulina in a strongly curved outdoor tubular photobioreactor. Appl. Microbiol. Biotechnol., 45:18-23). A disadvantage of pipe systems is that the volume within the photobioreactor is relatively small relative to the floorage or the area required by the system for the installation thereof, the so-called footprint. The pipe diameter must be kept relatively small in order for the light to reach the microorganisms in the part of the pipe located farthest away from the light source. Another disadvantage is that the flow in a pipe system is laminar. This may be remedied somewhat by means of curved pipes, in which the flow will be more turbulent.
Patent document US 2008274494 discloses a photobioreactor made of a transparent, flexible polymer material, for example polyethylene. The photobioreactor is suspended downwards from a rack in the form of a long, relatively wide and thin bag. Further, the bag is provided with internal flow deflectors in order to create turbulence when the algae cultivation medium flows down through the bag. The internal flow deflectors will also keep the walls of the bag together, whereby the bag does not bulge when being filled with liquid. The applicant's own patent document WO 2005121309 discloses a photobioreactor in the form of a flat, suspended bag with channels formed in the bag. Patent document U.S. Pat. No. 5,534,417 discloses a photobioreactor consisting of a series of pipes suspended downwards from a rack.
Patent document U.S. Pat. No. 5,981,271 discloses an apparatus for cultivating algae outdoors, wherein the algae reactor is a flat-lying chamber with a drop of approximately 3%. The depth of the chamber is approximately 5 cm.
Hereinafter, a cultivation fluid implies a liquid composed of components selected from the group consisting of: freshwater, brackish water, sea water, salt solution, bacteria, phototrophic bacteria, cyanobacteria, unicellular eukaryotic algae, multicellular eukaryotic algae, dinoflagellates, euglena, nutrient salts, gases in dissolved form, gases in non-dissolved form, minerals, trace elements, vitamins, acidity regulators, chelators, surfactants, antibiotics and thickeners.